Various light emission elements, such as the incandescent lamp developed by Edison, a fluorescent lamp, an LED (Light Emitting Diode), and an organic EL (electroluminescence), etc. exist in our life. They each have advantages and disadvantages, and they have been used according to a variety of applications including lights, televisions, and displays, etc. Therein, an LED using a compound semiconductor has high quantum efficiency where current is converted into light, so that it is used for a light emission element with high luminance and low power consumption. In these days, since crystal growth techniques of compound semiconductors have been progressing, blue light-emitting diodes which have been thought impossible in the twentieth century have been coming into wide use. For instance, they have been put to practical use such as in traffic lights. Since it has become possible to achieve blue, it has become possible to express three primary colors by using an LED with compound semiconductors. As a result, it has been possible to display white by combining a compound semiconductor and a phosphor material.
Therefore, an LED with a compound semiconductor becomes widespread as a light source for the back-light of a liquid crystal display of a cellular phone. An LED with a compound semiconductor has the advantage of high performance, such as high quantum efficiency and low power consumption, etc. However, since the compound semiconductor substrate is expensive and the manufacturing cost becomes high, in general, it is not suitable for a large surface area. However, an LED with a compound semiconductor can be used for a large display which is seen in public facilities such as train stations and airports. For applications in displays and lights, the LED which has gained attention recently is an organic EL element (or it may be abbreviated as an organic LED and OLED). Research on organic EL elements has explosively progressed since the latter half of the 1980's. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. (1987), vol. 51, pp. 913-915 written by C. W. Tang became the seminal work and they successfully obtained high efficiency emission by carrying current in a structure where electrodes are formed on the upper and lower surfaces of an organic thin film. However, the organic EL element had a short lifetime at first, and it was not put to a practical use. Since organic semiconductor materials, element structures, and sealing technologies have been progressing lately, installation in cellular phones has started and application in televisions has finally come to be discussed seriously.
As mentioned above, LED technology keeps progressing day by day, and research and development have been performed to develop an LED with low cost, high efficiency, low power consumption, large surface area, and high reliability. As a luminescent material for an LED, a semiconductor is used, and the material used most as a semiconductor is silicon. The percentage of elements existing in the vicinity of the surface of the Earth is known as a Clarke number; the Clarke number of silicon is 25.8% and it is the second most abundant element on the Earth next to oxygen with 49.5%. As a matter of course, the manufacturing cost is inexpensive and the technology for achieving high purity has been established, so that it is well known that silicon is used for a substrate material supporting electronics such as LSI (Large Scale Integration). Although silicon has various excellent features as a semiconductor material, it is difficult for it to emit with high efficiency which can be said to be the only disadvantage. The reason is that silicon is an indirect transition semiconductor in the bulk state. If silicon can be used as a luminescent material, it becomes possible to manufacture the light emission elements with high reliability inexpensively and on a large scale, so that it is needless to say that the industrial impact is enormous.
Then, a lot of research and development on making silicon an emitter has been carried out. In particular, much research has been carried out for improving the emission efficiency by the quantum confinement effect which has been proposed by L. T. Canham in L. T. Canham, Appl. Phys. Lett. (1990), vol. 57, pp. 1046-1048. The quantum confinement effect is an effect where the electron state changes with a low-dimensional structure such as a porous silicon and a nanocrystalline silicon, and silicon which does not emit in the bulk state becomes an emitter of light according to the quantum confinement effect when it has a low-dimensional nanostructure. Actually, there are many reports in which it emits with high efficiency by photoluminescence. For instance, in D. J. Lockwood, Z. H. Lu, and J. M. Baribeau, Phys. Rev. Lett. (1996), vol. 76, pp. 539-541, highly efficient photoluminescence was observed in a stacked structure of an amorphous silicon and silicon dioxide.
However, there is a problem that hardly any current is carried when it is made into a low-dimensional nanostructure. This is caused by the fact that the surface of silicon is easily oxidized and it becomes silicon dioxide which is an insulation material with a large band gap when it is oxidized. It is necessary to make it a low-dimensional nanostructure when silicon which does not emit in the bulk state is made to emit. The emission efficiency increases as it becomes a lower dimensional fine structure, but since it makes it has been made easy to cover the surrounding of the silicon nanostructure with an insulator film, it becomes difficult to flow a current therein. Since this is an intrinsic problem, it has been thought that it is very difficult to solve this dilemma. Because a conventional device carries the current perpendicular to, for instance, the stacked structure of the ultrathin silicon thin film and the silicon dioxide, carriers could be injected only by tunneling the silicon dioxide which is an insulator film. Forming p-type and n-type electrodes over the upper and the lower surfaces is a common structure of an LED, in which a compound semiconductor and an organic semiconductor are used, and a silicon LED with such a structure has terribly low efficiency.
Our group developed a device structure where this essential dilemma is easily solved when an advanced fine processing technique for silicon is used. It is a horizontal current injection type ultrathin film single crystal silicon light emission element disclosed in JP-A-2007-294628. This element forms a two-dimensional nanostructure by making the single crystalline silicon, which has a (100) face as the surface crystalline structure over an SOI (Silicon-On-Insulator) substrate, partially thin by using an oxidation process. The two-dimensional single crystalline silicon thin film having the quantum confinement effect is connected to a thick silicon electrode which is highly doped not through the silicon dioxide insulator film. Because it is connected directly, it is possible to inject current with high efficiency. As a result, a light emission element with high efficiency using silicon as a luminescent material could be successfully achieved. The silicon element has originally a planar structure and it is suitable for making it into a planer state. Therefore, in an LED, a device structure more suitable for a silicon LED can be designed by taking an idea of making it into a planar structure to which a general vertical structure is rotated by 90 degrees.